1. Field of the Invention
The present invention relates to a method for improving the earliness of plants, in particular higher plants. The present invention also relates to a method for lowering the content of nitrates stored in plants, in particular in the leaves, where appropriate.
Cultivated species have a reproductive cycle whose duration often limits utilization in the northern regions. In effect, it has to be possible for harvesting of the species to be performed before the return of adverse weather conditions. In many instances, ripening of the harvested organs and the seeds cannot be obtained in good time, and makes it necessary to harvest before maturity, or jeopardizes the crop. Thus, many species such as soybean are cultivated only below certain latitudes for this reason. Moreover, species already cultivated in northern zones, or at altitude, would gain by having a shorter cycle for the same reasons.
This type of problem is traditionally remedied either by using artificial cultivation conditions (greenhouse cultivation), a method which can be exploited for market garden crops, or by selecting for enhanced earliness. A gain in earliness may be obtained either by shortening the duration of the vegetative growth phase, or by accelerating floral induction, or lastly by facilitating the ripening of the fruits or seeds to be harvested. In general, the duration of the vegetative growth phase appears to be controlled by a complex set of genes, and behaves as a quantitative character. There is no indication of causal link between this duration and a particular aspect of the plant's metabolism.
The objective of the present invention is to provide a method that enables the duration of the vegetative phase to be shortened, and hence a gain in earliness to be obtained.
Hence, in the present application as well as in the technical field of the present invention, "early" is understood to denote varieties for which the time elapsing between planting of the seed and the subsequent harvesting is reduced. An "enhanced earliness" implies a shorter duration of the growth phase of the plant, which leads to a flowering and a ripening of the fruits or seeds to be harvested which occur further ahead in time than is normally the case.
Another objective of the present invention was to lower the nitrate content of certain plants, in particular in the leaves. The high nitrate level can, in effect, induce risks to health, as well as displeasure from the standpoint of the organoleptic properties of certain plants, especially for spinach, lettuce or carrot. For this reason, the nitrate contents in edible plants are now subject to control in many countries.
2. Description of the State of the Art
Nitrate reductase is a key enzyme which is known to come into play in the first step of nitrate assimilation in plants.
Nitrate is the most important source of nitrogen for higher plants. Nitrate is absorbed by the roots, transported to various tissues of the plant and then reduced to aqueous ammonia in two steps. The first step requires the enzyme nitrate reductase (NR), which catalyzes the reduction of nitrate to nitrite in the cytoplasm. In a second step, the nitrite is then reduced in the chloroplast by nitrite reductase. The reduction of nitrate is considered to be a major controlling step in nitrate assimilation, and it has been studied in detail in higher plants (Wray, 1986). NR is a homodimer bearing three cofactors, namely FAD, cytochrome b.sub.557 and a molybdopterin cofactor (Campbell, 1988).
Introduction of the NR gene into a plant has been proposed for modifying the characteristics of nitrate assimilation by plants on an exploratory or speculative basis, but it has not actually been possible to find any real application hitherto.
In any case, the use was always limited to the modulation of nitrate assimilation over time, that is to say according to the stage of development of the plant, or in space, that is to say in order to further assimilation in the roots, tubers or leaves.